Method of growing single crystals of ba2 zn2 fe12 o22



y 1968 T. R, AU COIN ETAL 3,384,449

METHOD OF GROWING SINGLE CRYSTALS OF B11 Zn Fe 0 Filed July 7, 1965 PULL I NC & ROTATING MECHANISM GROWING FERRITE CRYSTAL CONTROL THERMOCOUPLR QEBRICK PLUG FURNACE IO INVENTDRS,

I J I, NE .l APUUAVHE I: U A

n R STR AR MMH o T N RM T A ATTORNEYS United States Patent 3,384,449 METHOD OF GROWING SINGLE CRYSTALS 0F Ba Zrl Fe O Thomas R. Au Coin, Eatontown, Robert 0. Savage, Jr.,

Neptune City, and Arthur Tauber, Elberon, N.J., assignors to the United States of America as represented by the Secretary of the Army Filed July 7, 1965, Ser. No. 470,301 2 Claims. (CI. 23-51) ABSTRACT OF THE DISCLOSURE Single crystals of the formula Ba Zn Fe O are grown from a charge containing boron oxide, barium carbonate, zinc oxide and iron oxide. The charge is placed in a platinm crucible set in a vertical tube furnace and the furnace heated until the crucible containing charge attains a temperature of approximately 1150 C. to obtain a melt of the charge. A seed crystal attached to a crystal puller is then lowered until the said crystal makes contact with the melt. Then, the crystal is rotated, and while rotating, the crystal is slowly withdrawn from the melt while cooling the furnace at a constant rate.

The invention described herein may be manufactured and used by or for the Government for governmental purposes without the payment of any royalty thereon.

This invention relates to a method of growing hexagonal single crystals of the formula Ba Zn Fe O Heretofore, hexagonal single crystals corresponding to the above formula have been grown by heating a suitable charge such as a mixture of barium carbonate, zinc oxide, ferric oxide, and boron oxide in a platinum crucible contained in an electric resistance furnace at a temperature of about 1300 C., and then crystallizing the compound by slowly reducing the temperature followed by recovery of the crystals by treating the solidified melt with hot dilute nitric acid solution. The above described method was not entirely satisfactory in that the crystal grown is not large enough for its advantageous adaptation for use in various microwave devices such as its adaptation for use in a power limiter. Moreover, the single crystals have not been found to be mechanically stable and the single crystals include flux and fissures. Then too, the crystals cannot be grown oriented with respect to a specific crystallographic axis nor can they be annealed without being subject to the external pressures of a freezing melt. It has also been found that large platinum crucibles are required to grow large single crystals according to the aforementioned method. This increases the cost of the equipment thus making this method less desirable.

The general object of this invention is to provide a method of growing hexagonal single crystals of the formula Ba Zn Fe O A more specific object of this invention is to provide such a method which will avoid the disadvantages that are found with the above described method.

It has now been found that the above mentioned objectives can be attained and very desirable hexagonal single crystals of the formula Ba Zn Fe O grown by the following method. According to the method, a charge is first prepared containing barium carbonate, zinc oxide, iron oxide and boron oxide. Included in this charge is the fluxing agent, x BaO.B O where x may vary from 1.8 to 2.2 moles of R210 to one mole of B 0 In the charge, the starting ingredients that form the single crystal, Ba Zn Fe o are present in the amount from 70 weight percent of the total charge to 30 weight percent of the total charge Whereas the fluxing agent is present in the 3,384,449 Patented May 21, 1968 amount from 30 weight percent of the charge to 70 weight percent of the charge. The charge is placed in a platinum crucible and the crucible set in a vertical tube furnace. The furnace is then heated until the platinm crucible containing charge attains a temperature of approximately 1150 C. to obtain a melt of the charge. A seed crystal, such as a piece of hexagonal ferrite, which has been attached to a crystal puller is then carefully lowered until the seed crystal makes contact with the melt. The crystal is then rotated at 30 to 180 revolutions per minute and while being rotated, is withdrawn from the melt at a rate ranging from 0.0005 inch to 0.005 inch per hour while cooling the furance at a constant rate ranging from 0.l0 C. to 1.0 C. per hour. This slow cooling of the melt causes a very small amount of hexagonal single crystal of the formula Ba Zn Fe o to be precipitated from solution onto the seed crystal. The seed now grows larger at a rate governed only by the furnace cool down speed. The only limitations on crystal size are the size of the crucible and the allowable temperature drop before the Ba Zn Fe O phase is depleted.

The method of the invention can be best understood by referring to the drawing wherein there is shown a schematic view partially in cross section of apparatus that can be used to carry out the invention.

Referring to the drawing, 10 is a vertical tube furnace which is stopped in its lower opening with a firebrick plug 12, and in its upper opening with an alumina plug 14. Resting on the firebrick plug 12, is a platinum crucible 16 containing a charge 18. Extending vertically through alumina plug 14, is a crystal pulling and rotating mechanism 20 and a temperature controller programmer 22. A seed crystal 24 is attached to the lower end of the crystal pulling and rotating mechanism 20.

In carrying out one embodiment of the invention, the charge 18 is prepared from 17 grams boron oxide (B 0 111 grams barium carbonte (BaCO 7 grams zinc oxide (ZnO) and 41 grams ferric oxide (Fe O The charge is placed in the platinum crucible 16 and the crucible set in the vertical tube furnace 10, as shown in the drawing. The furnace is then heated until the crucible containing charge attains a temperature of approximately 1150 C. causing the charge to change to a melt of a saturated solution of Ba Zn Fe O in 2 132103 0 The seed crystal 24, a piece of hexagonal ferrite, is then lowered by the crystal pulling and rotating mechanism 20, until it makes contact with the melt as shown in the drawing. The crystal is then rotated at revolutions per minute and while being rotated is withdrawn from the solution at a rate of 0.003 inch per hour while the temperature controller programmer cools the furnace at a rate of 0.25 C. per hour. This slow cooling of the melt causes a very small amount of hexagonal single crystal of the formula Ba Zn Fe o 26 to be precipitated from solution onto the seed crystal as shown in the drawing. The single crystal now grows larger at a rate governed only by the furnace cool down speed. A large hexagonal single crystal is obtained which is about 2 to 3 times the size of the largest crystal that could be attained by the previously discussed method. The hexagonal single crystal grown according to the invention is also found to have improved mechanical stability, is free of flux inclusions and fissures, and can be annealed without being subject to the external pressures of a freezing melt. The hexagonal single crystal can also be grown oriented with respect to a specific crystallographic axis. Moreover, smaller platinum crucibles may be used in growing large crystals, thereby decreasing the cost of equipment. It is also possible to machine spheres of over 200 mils in diameter from the hexagonal single crystals grown according to the invention. Hexagonal single crystals grown according to the previously described method yielded at best machined spheres of 60 to 70 mils in diameter. This difference is most significant in the case where it is desired to use the single crystal in a power limiter where the requirement is a machined sphere of over 100 mils in diameter.

Various modifications of the invention method can be made. For example, other elements such as manganese can be substituted for the Zinc as desired. The seed crys tal can also be positioned at or beneath the surface of the solution and not withdrawn as it is growing. Then, it is also possible to add an excess of charge to produce a supersaturated solution and then discontinue the cool down of the furnace. The resulting vertical temperature gradient will cause hexagonal single crystals to crystallize from solution onto the seed crystal. In the instance where a supersaturated solution is produced, the seed crystal can also be rotated at or beneath the surface of the supersaturated solution while maintaining a constant temperature.

The foregoing description is to be considered merely as illustrative of the invention and not in limitation thereof.

What is claimed is:

1. Method of growing hexagonal single crystals of the formula Ba Zn Fe o including the steps of (1) preparing a charge containing boron oxide, barium carbonate, zinc oxide, and iron oxide, wherein said charge includes from 30 to 70 weight percent of the total charge of the fiuxing agent x BaO.B O wherein x varies from 1.8 to 2.2 moles of BaO to one mole Of B203,

(2) placing the charge in a platinum crucible set in a vertical tube furnace,

(3) heating the furnace until the crucible containing charge attains a temperature of approximately 1150 C. to obtain a melt of the charge,

(4) lowering a hexagonal ferrite seed crystal attached to a crystal puller until the seed crystal makes contact with the melt, and

(5) rotating the seed crystal at 30 to 180 revolutions 4 per minute, and while rotating, withdrawing the seed crystal from the melt at the rate of 0.0005 inch to 0.005 inch per hour while cooling the furnace at the rate of 010 C. to 1.0 C. per hour.

2. Method of growing hexagonal single crystals of the formula B21 Zn Fe O including the steps of (1) preparing a charge containing 17 parts by weight boron oxide, 111 parts by weight barium carbonate, 7 parts by Weight zinc oxide, and 41 parts by weight iron oxide,

(2) placing the charge in a paltinurn crucible set in a vertical tube furnace,

(3) heating the furnace until the crucible containing charge attains a temperature of approximately 1150 C. to obtain a melt of the charge,

(4) lowering a hexagonal ferrite seed crystal attached to a crystal puller until the seed crystal makes contact with the melt, and

(5) rotating the seed crystal at 90 revolutions per minute, and while rotating, withdrawing the seed crystal from the melt at the rate of 0.003 inch per hour while cooling the furnace at the rate of 025 C. per hour.

References Cited UNITED STATES PATENTS 3,117,934 1/1964 Linares 23 51 3,149,910 9/1964 'Iauber et a1 23-50 3,150,925 9/1964 Gambino 23-50 OTHER REFERENCES Laudise et al.: Journal of Applied Physics," Supplement to vol. 33, No. 3, March 1962, pp. 1362-1363.

T auber et al.: Journal of Applied Physics, Supplement to vol. 33, No. 3, March 1962, pp. 1381-1382.

OSCAR R. VERTIZ, Primary Examiner.

H. T. CARTER, Assistant Examiner. 

